Certify Pi Subagents and prepare v0.3.6

[regenrek]

Why:

• Promote the proven Pi Subagents workflow to certified, applyable support.
• Prepare release-owned artifacts for v0.3.6.

How:

• Certify Pi validation, catalog, bundles, onboarding, and generated lifecycle coverage.
• Regenerate release catalog, bundle, and fixture inventories.

Tests:

• cargo test --lib
• pnpm site:test
• pnpm site:test:parity
• cargo run --quiet -p xtask -- release verify --inventory-only